1. Field of the Invention
The present invention relates to a lens driving device mounted in a lens tube of an optical machine such as a video camera and configured to move a lens frame to which a lens (or lens assembly) is fixed, in the direction of an optical axis of the lens along first and second guide shafts arranged parallel to the optical axis.
2. Description of the Related Art
There is an optical device configured to smoothly move an optical element frame mounted to the optical device along an optical axis without trembling an image even if an engagement portion of the optical element frame has deviation factors. This optical device is disclosed in Japanese Patent Laid-open Publication No. H06(1994)-174992.
As shown in FIG. 1, the optical device 100 comprises an optical element 101, an optical element frame 102, a first guide shaft 103, a second guide shaft 104, a rack member 105, a motor 106, a feed screw 107 and a torsion spring 108.
The optical device 100 moves the optical element frame 102 to which the optical element 101 is fixed, in the direction of an optical axis K′ of the optical element 101 along the first and second guide shafts 103 and 104 arranged parallel to the optical axis K′. The rack member 105 is mounted swingablly to the optical element frame 102 below the first guide shaft 103 and capable of swinging in a direction perpendicular to the optical axis K′. The rack member 105 has a rack piece 105a and a rack biasing piece 105b which extend downward from an upper portion thereof and are arranged at a distance. The rack piece 105a is provided with rack teeth 105a1 on the inner surface thereof. The feed screw 107 is fixed to an output shaft of the motor 106 at one end portion thereof. The feed screw 107 is inserted between the rack piece 105a and the rack biasing piece 105b. The rack biasing piece 105b is pressed toward the feed screw 107 by biasing force of the torsion spring 108, which allows the feed screw 107 to mesh with the rack teeth 105a1 while the feed screw 107 is sandwiched between the rack piece 105a and the rack biasing piece 105b. 
As shown in FIG. 2, an abutting face 105b1 of the rack biasing piece 105b leans at an angle α° with respect to the rack teeth 105a1. Under this configuration, when the feed screw 107 meshes with the rack teeth 105a1 by pressing the rack biasing piece 105b toward the feed screw 107 by the biasing force of the torsion spring 108, sandwiching force acts from the rack piece 105a and the rack biasing piece 105b to the feed screw 107 to generate reaction force in an arrow direction 109.
Therefore, even if an engagement portion of the optical element frame 102 has deviation factors due to eccentricity and/or warpage of the feed screw 107, the optical element frame 102 can move along the optical axis K′ without trembling by setting the optical element frame's own weight and the leaning of the abutting face 105b1 such that the sum of the optical element frame's own weight and the reaction force generated in the arrow direction 109 is larger than trembling force (friction force) acting between the optical element frame 102 and the rack member 105.
However, if the accuracy of leaning of the abutting face 105b1 is low, overload is added to the feed screw 107 when the feed screw 107 is sandwiched between the rack piece 105a and the rack biasing piece 105b. This brings a problem that the optical element frame 102 can not smoothly move along the optical axis K′.
Further, in the optical device 100, there is no consideration for leaning of the optical element frame 102, which will occur at a time when the feed screw 107 rotates in a clockwise direction or an anticlockwise direction, due to lead angles of the rack teeth 105a1 and the feed screw 107.